1. Field of the Invention
This invention relates to a friction member to be used for a brake pad of a vehicle and the like.
2. Description of the Related Art
Conventionally, for a friction member to be used for a brake pad of a vehicle, various properties such that it has large braking force and is excellent in anti-wearing character, braking force of it is hardly changed even by change of temperature and pressure and is stable (anti-fading character), and it is not invaded by a water and an oil and has large material strength, are required.
As a friction member which can relatively satisfy above requirements, the friction member composed of a fiber-like substrate, bonding agent and a filling agent has been widely used. This friction member has blow holes of 10 to 20 volume %, and various sizes of blow holes from 0.01 xcexcm to few tens of microns are distributed in the friction member. Corresponding to high qualification of the vehicle such as high speed, it is required for the friction member to improve the anti-fading character and to reduce attacking character to a mating member i.e. rotor (referred as xe2x80x9cmate attacking characterxe2x80x9d hereinafter) for preventing brake vibration.
Generally, the anti-fading character and the mate attacking character are contradictory to each other, in other words, the friction member excellent in the anti-fading character has high mate attacking character, and the friction member low in the mate attacking character does not have sufficient anti-fading character. That is to say, in the conventional friction member excellent in the anti-fading character, gas generated in a resin component of the friction member upon fading is escaped into the relatively large size blow holes in the friction member having hole diameter larger than 1 xcexcm to thereby increase the anti-fading character. However, worn powders generated due to friction between the pad and rotor when the wheel is rotating idly may be held in those blow holes of large size to operate as a grinding particles, so that the rotor is worn.
In view of the above, in the friction pad in which hole rate of the blow holes having hole diameter equal to or larger than 3 xcexcm is decreased to restrict the mate attacking character, by taking relation between the blow hole rate and hole diameter of the whole friction member into consideration, large numbers of smaller blow holes from 0.1 to 0.5 xcexcm are formed. However, such friction pad can not maintain predetermined anti-fading character in a mounting road sports travelling such as rally travelling, although it can maintain the anti-fading character in the normal travelling.
The present invention is made, in view of the above circumstances, and intends to provide the friction member which is excellent in the anti-fading character, but is low in the mate attacking character.
In order to overcome the above mentioned disadvantage, inventors of the present application has studied about the blow hole rate of the friction member to find out the fact that decreasing the numbers of the large diameter blow holes which may hold the friction powders therein and increasing an accumulated blow hole rate of blow holes having hole size which can escape gas generated upon fading are effective for solution of the problem. Here, xe2x80x9cblow hole ratexe2x80x9d means the rate of void portion of the blow holes relative to an artificial whole volume of the friction member and is expressed by volume %.
The present invention is featured by that in the friction member composed of the fiber-like substrate, bonding material or agent, and filling material or agent, an accumulated blow hole rate of the blow holes having hole diameter equal to or more than (not less than) 3.5 xcexcm is selected to be not larger than 2 volume %, and an accumulated blow hole rate of the blow holes having hole diameter from 0.5 to 3.5 xcexcm is selected to be not less than 3 volume %. Here, the significance of the above critical values (2 volume % and 3, 5 volume %) will be explained. If the accumulated blow hole rate of the blow holes having hole diameter not less than 3.5 xcexcm increases over 2 volume %, the mate attacking character of the friction member will be also increased, whereas if the accumulated blow hole rate of the blow holes having hole diameter from 0.5 to 3.5 xcexcm decreases below 3 volume %, the anti-fading character will be also decreased.
In the present invention, at a part or portion of the friction member located between a surface and a point of 8 mm depth in thickness direction, it is preferable that an accumulated blow hole rate of the blow holes having hole diameter equal to or more than (not less than) 3.5 xcexcm is selected to be not larger than 2 volume %, and an accumulated blow hole rate of the blow holes having hole diameter from 0.5 to 3.5 xcexcm is selected to be not less than 3 volume %. That is to say, provision or distribution of the blow holes satisfying the above volume % in the part of friction member located from the surface to the point of 8 mm, depth will facilitate escapement of the gas generated upon fading sufficiently. The blow hole rate is controlled by adjusting a forming time and a forming temperature in a heat forming of the friction member.
For the bonding agent, a phenol resin, epoxy resin or polyimide resin which has been used in the conventional friction member can be used, like wisely.
For the filling agent, organic and inorganic lubricating agent and filling agent which have been used in the conventional friction member can be used. They are, for example, carbon black, graphite, barium sulfate, calcium carbonate, silica, rubber particles, or cashew dust.
It is preferable for the surface of friction member to be subjected to a scorch treatment. Here xe2x80x9cscorch treatmentxe2x80x9d is the treatment in which a friction surface is baked in advance to prevent occurrence of the fade due to resolved gas generated from the organic substrate contained in the friction member. The friction surface subjected to the scorch treatment has larger blow hole rate than an inner portion not subjected to the scorch treatment to allow gas escapement.
The scorch treatment is performed over the portion of the friction member located from 2 to 4 mm from the friction surface. In a portion located from 2 to 8 mm from the friction surface and not subjected to the scorch treatment, it is preferable to select the accumulated blow hole rate of the blow holes having hole diameter not less than 3.5 xcexcm to be not more than 2 volume %, and to select the accumulated blow hole rate of the blow holes having hole diameter from 0.5 to 3.5 xcexcm to be not smaller than 3 volume %.
It is enough for the friction member that only the portion located 2 to 4 mm depth from the friction surface is subjected to the scorch treatment, because the portion located 2 to 8 mm depth from the friction surface and not subjected to the scorch treatment will be sequentially subjected to the scorch treatment by baking of the friction member due to heat of the hard braking under travelling, while the friction surface subjected to the scorch treatment is worn.
The fiber-like substrate preferably includes a metallic fiber and an inorganic fiber by the rate not less than 25 wt %, when the friction member is assumed to have 100 wt %. The metallic fiber and inorganic fiber form and maintain framework in the friction member so that the blow holes formed therein are not collapsed. For this reason, if rate of the inorganic fiber is smaller than 25 wt %, the blow holes in the friction member are collapsed to reduce the rate of void holes, so that anti-fading character of the friction member is decreased.
For the metallic fiber, a steel fiber, stainless steel fiber, titanic or copper fiber can be used, and for the inorganic fiber, a rock wool, glass fiber, silicate fiber, aluminum fiber, carbonate fiber, calcium silicate fiber or potassium titanate fiber can be used.
The fiber-like substrate preferably contains, in addition to the metallic fiber and inorganic fiber of not less than 25 wt %, an organic fiber such as flax, cotton or aromatic polyamide fiber. In other words, it is enough for the substrate to contain the metallic fiber and the inorganic fiber by not less than 25 wt %, which enables to contain the organic fiber other than above two kinds of the fibers.
The friction member can be manufactured by the normal manufacturing method thereof. That is to say, raw materials such as the fiber-like substrate, bonding agent and filling agent are sufficiently mixed in the predetermined rate, then formed into the predetermined shape by the heat forming, and then subjected to the heat treatment.
According to the friction member of present invention, not only the mate attacking character is maintained low by reducing the number of blow-holes into which the friction powders are held, but the anti-fading character is maintained high by increasing the number blow holes which allow gas generated upon fading to escape. Further, on account of the metallic fiber and the inorganic fiber mixed into the friction member the blow holes are hardly collapsed so that the low mate attacking character and high anti-fading character can be maintained.